Study On BDS Differential Code Bias Estimation And Analysis Of Ionospheric TEC Model

Ionospheric delay is one of the main errors in GNSS positioning,which directly affects the accuracy of GNSS positioning.Therefore,it is of great significance to model and monitor the ionosphere.The Global ionospheric Maps are the main products of ionospheric modeling.It can provide information about the temporal and spatial variations of ionospheric total electron content.In order to accurately reflect the variation characteristics and spatial distribution of ionospheric TEC,the measurement and prediction of TEC is an important topic in ionospheric studies.In addition,although differential code bias is an accessory product of global ionospheric grid products,it not only directly affects the extraction of ionospheric TEC,but also has a significant impact on navigation,positioning and timing.Especially with the rapid development of GNSS,various navigation systems begin to broadcast multi-frequency and multi-channel signals,which provide new development opportunities for navigation and positioning,but also derive more types of DCBs.Based on this,this thesis mainly studies BDS DCB estimation and ionospheric TEC prediction model,and the main work and research results are as follows.(1)In this study,we proposed a BDS DCB estimation method based on altitude angle,the distance between the station and the satellite and station latitude.Then we analyzed the influence of different weighting schemes on BDS DCB estimation and ionospheric modeling.According to the multi-frequency and multi-channel characteristics of BDS satellites,several types of DCBs of BDS satellites were estimated,and the accuracy and stability of each type of DCBs were evaluated.Experiments showed that the DCB estimation method based on mixed weights can improve the calculation accuracy of DCB at high latitude stations.Compared with the traditional height Angle model,the root-mean-square error of ionospheric modeling based on mixed weight model was reduced by 0.88 TECU,which improves the accuracy of ionospheric modeling.(2)By performing epoch by epoch estimation on BDS DCB,the short-time variation characteristics of receiver and satellite DCB were analyzed,and the influence of DCB correction on pseudometric single point positioning was verified by experiments.The experimental results showed that adding DCB correction can significantly improve the positioning accuracy of SPP.Both the addition of short-term DCB correction and the DCB correction provided by CAS showed an improvement effect of about 45%.In addition,the short-time DCB correction and the DCB correction provided by CAS have advantages and disadvantages of each other in SPP positioning accuracy at different stations,and the difference is in centimeter level.The average improvement effect of short-time DCB correction is slightly better,but not obvious,almost roughly equivalent.(3)this thesis combined EWT and LSTM to forecast ionospheric TEC.The prediction accuracy of EWT-LSTM model,LSTM model and Holt-Winters model were compared,and the influences of different latitude regions and geomagnetic activities on the prediction accuracy of ionospheric TEC were analyzed.The experimental results showed that compared with the LSTM model and Holt-Winters model,the prediction accuracy of EWT-LSTM model was significantly improved and has higher accuracy.The intensity of geomagnetic activity affects the accuracy of model prediction.Generally,the more intense the geomagnetic activity,the worse the prediction accuracy of the model.And the prediction accuracy of the forecast model is different in different latitude regions.However,no matter in the period of magnetic calm or magnetic storm,the forecast RMSE of the three models decreased with the increase of latitude,that is,the forecast accuracy would decrease with the increase of latitude.This thesis has 39 figures,21 tables,and 91 references.